1. Field of the Invention
The present invention relates to an optical scanning apparatus, for use in, for example, a microscope, for scanning an object which is to be inspected.
2. Description of the Related Art
FIG. 5 shows an optical-scanning type microscope having a optical scanning system which is well-known in the art. In the optical scanning system shown in FIG. 5, a laser beam 2 emitted from a laser source 1 is expanded by a beam expander 3 to a desired diameter. The expanded laser beam passes through a beam splitter 4, and applied to a first optical deflecting system 5. The direction of the beam is changed by the first optical deflecting system 5 and the beam spot formed on the sample 12 (to be described later) is moved in a direction (Y direction) in accordance with the operation of the first optical deflecting system 5.
The first optical deflecting system 5 is located at a conjugate position with respect to the pupil of an objective lens which will be described later. The beam deflected by the first optical deflecting system 5 is applied to a second optical deflecting system 8 via pupil transmission lenses 6 and 7.
The second optical deflecting system 8 is located at a conjugate position with respect to the pupil of the objective lens and deflects the beam in an X direction which is perpendicular to the Y direction. The beam deflected by the second optical deflecting system 8 are applied to the objective lens 11 via a pupil transmission lens 9 and an image forming lens 10. The objective lens 11 forms a beam spot on a portion of the sample 12.
Since the first and second optical deflecting system 5 and 8 are located at positions conjugate to the pupil of the objective lens 11, when the optical beam is deflected in the X and Y directions by means of the optical deflecting systems 5 and 8, the beam spot moves on the sample 12 in the X and Y directions in a state that the optical axis is maintained. In other words, the sample 12 is scanned in the X and Y directions by the beam spot.
If the sample 12 is transparent, the beam transmitted through the sample 12 is applied via a condenser lens to an optical detector 14.
If the sample 12 is reflective, the beam reflected by the sample 12 returns along the path, reflected by the semi-transparent surface of the beam splitter 4, and applied via a converging lens 15 to an optical detector 16.
FIG. 6A shows an optical system from the optical deflecting system 5 to the objective lens 11. Since the pupil E1 of the objective lens 11 is conjugate with respect to the optical deflecting systems 5 and 8, the centers of the beams deflected by the optical deflecting systems 5 and 8 coincide with an on-the-axis main beam and pass the center of the pupil E1. The uppermost and lowermost lights L1 and L2 of the on-the-axis beam meet the uppermost and lowermost lights L3 and L4 of the off-the-axis beam in the pupil E1. Thus, the diameter of the laser beam at the pupil position is substantially the same as that of the pupil of the objecting lens 11 regardless of the deflection angles of the first and second optical deflecting systems 5 and 8.
However, since the above optical scanning system of the conventional type includes the pupil transmission lenses 6 and 7 between the first and second optical deflecting systems 5 and 8 in order to make the deflecting systems 5 and 8 conjugate the pupil E1 of the objective lens 11, the entire system is inevitably large.
Moreover, since the first and second deflecting systems 5 and 8 are fixed, the pupil of the objective lens which is conjugate with respect to the deflecting systems 5 and 8 by virtue of the pupil transmission lenses 6 and 7 is also fixed. Hence, when an objective lens is replaced with another lens having a different pupil position, the peripheral portion of the beam incident on the objective lens is off the pupil, resulting in limb darkening, since the peripheral portion of the beam does not pass the pupil.
The limb darkening phenomenon will be described with reference to FIG. 6B.
In the system shown in FIG. 6B, an objective lens 17 has a pupil at a different position from that of the objective lens 11 in FIG. 6A. Accordingly, the pupil E2 of the objective lens 17 is not in a position conjugate to the optical deflecting systems 5 and 8.
In this case, the center of the beam deflected by the deflecting systems 5 and 8 does not pass the center of the pupil E2 of the objective lens 17. The uppermost and lowermost lights L1 and L2 of the on-the axis beam do not meet the uppermost and lowermost lights 33 and 44 of the off-the axis beam at the pupil position E2.
FIG. 7A shows a state of the beam which passes the pupil E2 shown in FIG. 6B. As is shown in FIG. 7A, an on-the-axis beam 20 passes within the pupil 21 of the objective lens 17. However, a deflected off-the-axis light beam 22 passes off the pupil 21, and the shadowed portion of the beam 22, which is off the pupil 21, does not applied to the sample. The area of the shadowed portion increases in accordance with increase of the deflection angle, thereby causing a limb darkening phenomenon.
Further, in the conventional optical scanning system, the diameter of a beam incident on the optical deflecting systems 5 and 8 is unchangeable, although the pupils of objective lenses have various diameters. Therefore, the diameters of an on-the-axis beam 23 and an off-the-axis beam 24 may be greater than that of the pupil 21, as is shown in FIG. 7B. In this case, although no limb darkening occur, the beam cannot be used efficiently.